Method and device for manufacturing pellets of hot-melt ink

ABSTRACT

A method for manufacturing pellets of hot-melt ink which includes the steps of filling molten ink into a mold cavity defined by a first die and a second die of a mold, allowing the ink to cool down and solidify in the mold cavity, and heating at least one of the first and second dies for re-melting the surface of the ink pellet to facilitate its removal from the mold cavity.

BACKGROUND OF THE INVENTION

This application is a Divisional of application Ser. No. 10/079,556,filed on Feb. 22, 2002 and now U.S. Pat. No. 6,673,299, the entirecontents of which are hereby incorporated by reference and for whichpriority is claimed under 35U.S.C. § 120; and this application claimspriority of Application No. 01200691 filed in Europe on Feb. 23, 2001under 35 U.S.C. § 119.

FIELD OF THE INVENTION

The present invention relates to a method and a mold for manufacturingpellets of hot-melt ink.

BACKGROUND ART

Certain types of inkjet printers employ a so-called hot-melt ink i.e. awax-like ink material that is solid at room temperature and has amelting point in the order of, for example, 80 to 150° C. In theprinthead of the printer, the ink is heated above its melting point, sothat droplets of liquid ink can be expelled through the nozzles of theprinthead. In order to obtain a high quality of the printed image, theviscosity and hence the temperature of the molten ink in the printheadshould be maintained essentially constant. However, since the ink isconsumed in the course of the printing process, and the ink reservoiraccommodating the liquid ink within the printhead is preferably of alimited size, it is necessary to supply and melt solid ink while theprinter is operating. The latent heat required for melting the ink tendsto decrease the temperature in the ink reservoir. For this reason, it isdesirable that the amount of solid ink supplied to the ink reservoir isprecisely controlled and metered, and, to this end, it is advantageousthat the ink is supplied in the form of pellets having a predeterminedsize and shape, e.g. in the form of small spherical pills or pellets.

Since the hot-melt ink is a thermoplastic material, the pellets havingthe desired shape and size can be manufactured by means of a moldingprocess similar to injection molding processes known for manufacturingarticles from thermoplastic resins. The molding process however shouldbe adapted to the specific properties of hot melt ink, which are, incertain respects, different from those of thermoplastic syntheticresins. Since the amount of shrinkage which the hot-melt ink experienceswhen it is solidified is comparatively low, and since a certain amountof shrinkage can be tolerated because the final appearance of the moldedink pellets is not critical, it is not necessary to apply high lockingforces for keeping the mold closed during the molding process. On theother hand, since the hot-melt ink has a relatively high melting point,it tends to solidify immediately when it comes into contact with thewalls of the mold cavity. This effect and the fact that the surface ofthe ink pellet is somewhat tacky, even when the temperature has droppedbelow the melting point, increases the tendency of the pellet to adhereto the walls of the mold cavity. This makes it more difficult toreliably and reproducingly remove the molded pellet from the mold die.Especially when the upper and lower dies of the mold are symmetrical, asmust be the case for example when the pellet has a spherical shape, itis not predictable whether the pellet will adhere to the upper die or tothe lower die when the dies of the molds are separated. This tends toreduce the productivity of the molding process and/or necessitates theuse of complex mechanisms for ejecting the molded product from the die.

It is well known that the removal of a molded product from a die can befacilitated by employing a separating agent which reduces the adherencebetween the molded product and the walls of the mold cavity. In thiscase, however, a portion of the separating agent will inevitably bedispersed or diluted in the molten material, and this is not acceptablein the case of hot-melt ink because it deteriorates the quality of theink. For example, even minute particles of the separating agent, whendispersed in the ink, tends to clog the extremely fine nozzles of theprinthead or change the ink properties such as its surface tension orcrystallization point.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method formanufacturing pellets of hot-melt ink, in which the pellets can bereliably and reproducibly withdrawn from the mold cavity.

According to the present invention, this object is achieved by a methodcomprising the steps of:

-   -   filling the molten ink into a mold cavity defined between a        first die and a second die of a mold,    -   allowing the ink to cool down and to solidify in the mold        cavity, and    -   heating at least one of the first and second dies for remelting        only the surface of the ink pellet to be removed from the mold        cavity.

According to the present invention, the molded pellet is separated fromthe wall of the mold cavity by heating at least a portion of the mold,so that a surface layer of the pellet is remelted. This can be achievedwithin a very short time. Thus, it is possible to remove the pellet fromthe mold cavity efficiently and in a well-defined manner. Since it isnot necessary to employ a separating agent, the quality of the hot-meltink will not be degraded.

When the lower die of the mold is heated before the upper and lower diesare separated, it is possible to positively release the pellet from thelower die and to withdraw it from the lower die, taking advantage of thefact that the pellet tends to adhere to the upper die which is notheated. Then, the pellet is released from the upper die by any suitablemeans, thereby allowing the pellet to simply drop out of the upper die.The pellets dropping out of the upper dies may be collected by anysuitable collection means such as a chute which is brought in positionunderneath the pellets that have been withdrawn from the lower dies.

Further, it is possible to release the pellet from the upper die byheating the latter. In a preferred embodiment, the method comprises thesteps of first heating the lower die, then separating the upper andlower dies with the pellets adhering to the upper die, and heating theupper die, thereby allowing the pellet to drop out. The release of thepellet from the upper die may be assisted and accelerated by blowing airinto the runner hole of the upper die. As an alternative, an ejector pinmay be inserted through the runner hole. In this case, the ejector pinmay be arranged stationary, so it enters into the runner hole andengages the pellet adhered thereto when the upper die and the pellet arelifted from the lower die.

A mold for manufacturing pellets of hot-melt ink in accordance with themethod described above comprises first and second dies defining a moldcavity, wherein at least one of the first and second dies has a wallthickness which is smaller than half the diameter of the mold cavity. Ifthe mold cavity is not spherical, the wall thickness of the die issmaller than half the average diameter.

Due to the small wall thickness, the die has a very low heat capacity,such that the surface layer of the molded pellet can be remelted veryquickly by heating the die. The small heat capacity of the die has thefurther advantage that the molten ink in the mold cavity can be cooledand solidified more rapidly, so that the productivity of the moldingprocess is increased.

Preferably, both dies of the mold have a small wall thickness and hencea small heat capacity and are made of a material having a high heatconductivity, e.g. aluminium. Also stainless steel is useable if thewall thickness is small enough. In a preferred embodiment, the wallthickness of the dies is smaller than a quarter of the diameter of themold cavity. For example, if the mold cavity is spherical and has adiameter in the order of 10 mm, the wall thickness of the dies may be1.5 mm or less.

Rapid cooling and re-heating of the dies may be achieved in a verysimple manner e.g. by blowing cold and hot air or even a liquid againstthe dies. A number of other heating or cooling devices can be used.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described inconjunction with accompanying drawings, in which:

FIGS. 1 to 5 illustrate successive steps of a process for moldinghot-melt ink pellets and removing them from the mold cavity.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a group of three molds 10, each of which comprises an upperdie 12 and a lower die 14 each of which have a semi-spherical cup shapeand, together, define a mold cavity 16 which is filled with moltenhot-melt ink 18. The upper die 12 is integrally formed with a top flange20 and has a runner hole 22 formed in the center of the flange 20, sothat molten ink can be poured into the mold cavity 18 through a nozzle24.

The lower die 14 is essentially mirror-symmetric relative to the upperdie 12 and is supported on a bottom 26 formed integrally therewith. Thelower edge of the upper die 12 and the upper edge of the lower die 14are surrounded by circumferential flanges 28, 30 which are held in firmengagement with one another in order to sealingly close the mold cavity16.

When the ink 18 has been poured in, as is shown in FIG. 1, the molds aretransferred to a cooling stage illustrated in FIG. 2, where cold air 32is blown against the outer surfaces of the dies 12,14 from above andbelow, so that the ink in the mold cavities is cooled and solidified toform spherical pellets 34.

Then, the molds 10 are transferred to a first heating stage which isshown in FIG. 3. This heating stage comprises a heating block 36 havinga number of recesses 38 for accommodating the lower dies 14 of themolds. The recesses 38 have a flat bottom, which defines a large contactarea with the bottom flanges 26 of the lower dies 14. Hot air issupplied into a system of passages 40 formed in the heating block 36 andis evenly blown out against the circumferential walls of the lower dies14 of each mold 10, as indicated by arrow 42. The dies 12,14 of themolds 10 are made of aluminium and have relatively thin walls (at leastin the portion defining the mold cavity), so that their heat capacity islow, but their heat conductivity is high. As a result, the hot air blownagainst the walls of the dies 14 rapidly raise the temperature of thesedies, and surface layers of the pellets 34 facing the lower dies 14 arere-melted, so that the pellets 34 can easily be released from the lowerdies 14. However, since the upper dies 12 have not been heated, thesolidified material of the pellets 34 still adheres to the upper die 12.Since the heating block 36 is constantly maintained at a hightemperature (e.g. by the hot air passing therethrough), heating of thelower die 14 is accelerated by heat radiation and thermal contactbetween the block 36 and the bottom flange 26.

Then, as is shown in FIG. 4, the upper and lower dies of each mold 10are separated from one another, either by lifting the upper dies 12 orby lowering the heating block 36 and the lower dies 14. Since thepellets 34 stick to the upper dies 12, they are withdrawn from the lowerdies 14.

Finally, the upper dies 12 with the pellets 34 held therein aretransferred to a second heating stage shown in FIG. 5. This heatingstage comprises a heating block 44 which has essentially the sameconfiguration as the heating block 36 described above, but is arrangedin an inverted position so that the recesses 38 face downward foraccommodating the top flanges 20 of the upper dies 12. In addition tothe system of passages 40 for blowing hot air against the outer surfacesof the dies 12, the heating block 44 has another air supply system 46through which air can be blown with a suitable pressure into the runnerholes 22 of the dies 12. Again, by blowing hot air, indicated by arrows48, against the dies 12, surface layers of the pellets 34 are re-melted,so that the pellets will no longer adhere to the dies 12 but will dropdown into a chute 50. This process is assisted and accelerated byblowing pressurized air into the runner holes 22. Thus, the moldingprocess for manufacturing the pellets 34 is completed, and the upper andlower dies 12, 14 may be re-circulated for use in another molding cycle.

Although not shown in the drawings, the dies 12, 14 of the molds 10, thetotal number of which may be significantly larger than three, may bemounted to an endless conveyor in any known manner allowing to hold themolds 10 closed in the step illustrated in FIGS. 1 to 3 and to move theupper dies 12 and the lower dies 14 relative to one another in verticaldirection in the step illustrated in FIG. 4. Thus, the process describedabove lends itself to an efficient mass production of hot-melt inkpellets 34.

1. A mold for manufacturing pellets of hot-melt ink which consistsessentially of first and second dies which are essentially cup-shapedand have a wall thickness of smaller than 2.0 mm, said first and seconddies defining a mold cavity, said dies being separable from one another,at least one of said dies having a wall thickness which is smaller thanone-half the average diameter of the mold cavity, the upper one of saidfirst and second dies containing a single runner hole disposedsubstantially in the center of said upper die and having a sizesufficient for the introduction of ink for the manufacture of saidpellets into the mold cavity.
 2. The mold according to claim 1, whereinsaid dies are essentially cup-shaped and have a wall thickness smallerthan 2.0 mm.
 3. The mold according to claim 1, wherein the dies are madeof a metal or a metal alloy.
 4. The mold of claim 3, wherein the diesare made of aluminum or stainless steel.
 5. The mold of claim 3, whereina heating system is operatively associated with at least one of saiddies.
 6. The mold of claim 3, wherein the heating means includes meansfor blowing hot air against the outer surfaces of the dies.
 7. The moldof claim 3, wherein said runner is adapted for introducing blown airinto the mold cavity.
 8. The mold of claim 5 wherein the heating meansis a heating block containing a plurality of first and/or second dies ofrecesses for accommodating a plurality of molds.
 9. The mold of claim 1,wherein the first and second dies define a substantially spherical moldcavity.
 10. A mold for manufacturing pellets of hot-melt ink whichcomprises first and second dies defining a mold cavity, said dies beingseparable from one another, at least one of said dies having a wallthickness which is smaller than one-half the average diameter of themold cavity, the upper one of said first and second dies containing arunner hole of a size sufficient for the introduction of ink for themanufacture of said pellets into the mold cavity; and a heating systemis operatively associated with at least one of said dies, said heatingsystem being a heating block containing a plurality of first and/orsecond dies of recesses for accommodating a plurality of molds.
 11. Themold of claim 10, wherein the recesses have a flat surface which definesa large contact area to accommodate corresponding flanges on the dies.12. The mold of claim 1 wherein the mold cavity is completely sealedexcept for said runner hole.
 13. A mold for manufacturing pellets ofhot-melt ink which consists essentially of first and second dies made ofa metal or a metal alloy and defining a mold cavity said dies beingseparable from one another, at least one of said dies having a wallthickness which is smaller than one-half the average diameter of themold cavity, the upper one of said first and second dies containing asingle runner hole disposed substantially in the center of said upperdie and having a size sufficient for the introduction of ink for themanufacture of said pellets into the mold cavity, and a heating systemoperatively associated with at least one of said dies.
 14. The moldaccording to claim 13, wherein said dies are essentially cup-shaped andhave a wall thickness smaller than 2.0 mm.
 15. The mold according toclaim 13, wherein the dies are made of a metal or a metal alloy.
 16. Themold of claim 13, wherein the heating means includes means for blowinghot air against the outer surfaces of the dies.
 17. The mold of claim13, wherein said runner is adapted for introducing blown air into themold cavity.
 18. The mold of claim 13 wherein the heating means is aheating block containing a plurality of first and/or second dies ofrecesses for accommodating a plurality of molds.
 19. The mold of claim8, wherein the recesses have a flat surface which defines a largecontact area to accommodate corresponding flanges on the dies.
 20. Themold of claim 13 wherein the mold cavity is completely sealed except forsaid runner hole.